Wireless communication systems are known to include a plurality of communication units and wireless infrastructure. The communication units, which include mobile radios, portable radios, cellular telephones, personal digital assistants, computers with wireless modems, and so forth, communicate with each other via the wireless infrastructure. The wireless infrastructure includes communication resources, such as wireless communication channels and control channels, base stations, base station controllers, and mobile switching centers. The base stations are distributed throughout a geographical region to provide wireless coverage for that region. The base stations are often coupled to a base station controller that coordinates communications within that region. Base station controllers within various regions are typically coupled to the mobile switching center that coordinates communications throughout the plurality of regions.
Once a communication is established in a digital wireless communication system, a communication unit receives an audible input signal, e.g., speech, from its user. The communication unit encodes the audible input signal into a vocoded digital format, such as IMBE (Improved Multi-Band Excitation), and transmits the vocoded digital signal to the wireless infrastructure. The wireless infrastructure processes the received vocoded digital signal and provides the processed signal to the appropriate target communication units, i.e., the communication units that were intended to receive the communication. In today's digital wireless communication systems, the processing of received vocoded digital signals takes a few hundred milliseconds to well over a second to process and provide to the target communication units.
If the originating communication unit, i.e., the communication unit receiving the audible input, is in close physical proximity to one of the target communication units, audio feedback, also known as an echo, may occur. In close physical proximity, the microphone of the originating communication unit may receive the audible output from the target communication unit, thereby producing an echo. Because the microphone cannot distinguish between the original audible signal and the processed audible signal, the communication unit processes both in the same manner. As a result, all of the communication units involved in the communication hear the echo. In addition to the echo created by the equipment, the operator of the initiating communication unit hears the audible output of the targeted communication unit, which output occurs a noticeable time after the originate audible input signal was created. Such feedback can be quite distracting to the talking user as well as the listening users.
Currently, digital wireless communication systems do not incorporate echo cancellation circuitry to overcome the above mentioned problems. There are, however, echo cancellation circuits used in connection with telephony. Such echo cancellation is derived for electrical audio reflections, which are generated by mismatches and imperfections when converting four wire audio, e.g., from an isolated receiver/transmitter system, to a two-wire, full-duplex circuit, such as a telephone line. Such echo cancellation circuits may be either passive or dynamic. Passive echo cancellation is performed in a two- to four-wire hybrid combiner by way of phase inversion on the receiving path. The phase inversion cancels the audio energy that would otherwise be reflected back on the four-wire received port. Newer digital signal processing technologies enable the passive echo cancellation capabilities to be extended to variable processing, thereby providing dynamic echo cancellation. Dynamic echo cancellation circuits estimate the reflective delay and combines or subtracts a delayed version of the transmitted source audio, thus removing the echo component. While both implementations solve the problem in the time domain by performing wave form subtraction to remove the undesired electrical reflective audio, such echo cancellation circuits do not address the longer delay that occurs in a digital wireless communication system.
Therefore, a need exists for a method and apparatus that performs echo cancellation and/or attenuation in a digital wireless communication system.